1. Field of the Invention
The present invention relates to a frequency-switching oscillator; in particular, the invention relates to a frequency-switching oscillator capable of outputting signals of a plurality of frequencies, and a communication apparatus using the frequency-switching oscillator.
2. Related Art
Recently, in markets for radio communication, particularly, for portable telephones, multiple communication services are provided in operating frequencies different from each other. Under these circumstances, in order to enjoy multiple communication services by one communication apparatus, signal sources (oscillators) of plural different frequencies from each other are demanded in one communication apparatus. Apparatuses that can be considered to be suitable for this purpose include a frequency-switching oscillator capable of switching and outputting signals of multiple frequencies.
FIG. 6 shows a conventional frequency-switching oscillator. A frequency-switching oscillator 1 is configured of an oscillating circuit 2 and a buffer circuit 3. In the oscillating circuit 2, a resonant circuit 4 capable of switching to two resonant frequencies is connected to the base of the transistor Q1 provided as a first oscillating active element. The collector of the transistor Q1 is grounded in a radio-frequency (RF) band to be used via a capacitor C1 (in a high frequency manner), and the emitter thereof is connected to the ground via a resistor R1 and a capacitor C2 to which the emitter is connected in parallel. The base and the emitter of the transistor Q1 are connected to each other via a capacitor C3. The emitter of the transistor Q1 is connected to the buffer circuit 3 via a capacitor C4.
The expression that an element is grounded in an RF band to be used via a capacitor (in a high frequency manner) substantially means that the element is connected to the ground via, for example, a capacitor with relatively large capacitance which allows the impedance to become sufficiently small at an oscillation frequency, whereby, despite that the element is not directly grounded, the element is made to be in the same condition as in the case where the element is substantially grounded at the oscillation frequency, i.e., a radio-frequency (high frequency band). Although the capacitance is not very large, a capacitor which causes series self-resonance by its own capacitance and a parasitic inductance component so as to be of a low impedance may be used.
In the buffer circuit 3, the transistor Q2 provided as a second amplifying active element and the transistor Q1 of the oscillating circuit 2 are cascade-connected by connecting their respective emitters and collectors. Also, the base of the transistor Q2 is connected to the oscillating circuit 2 via the capacitor C4, the emitter thereof is grounded via the capacitor C1 in an RF band to be used, and the collector is connected to an outputting terminal 5 via a capacitor C5. In addition, the collector of the transistor Q2 is connected to the ground via a capacitor C6, and is connected to a power-supplying terminal 6 via an inductor L1 comprising of lumped constant line; and the capacitor C6 and an inductor L1 constitutes a matching circuit 7. In addition, the power-supplying terminal 6 is grounded in an RF band to be used via a capacitor C7.
Resistors R2, R3, and R4 are connected in series between the power-supplying terminal 6 and the ground. A node between the resistors R2 and R3 is connected to the base of the transistor Q2. A node between the resistors R3 and R4 is connected to the base of the transistor Q1, whereby bias voltage is supplied to the bases of the transistor Q1 and the transistor Q2. Among these elements, the capacitors C1 and C4 and the resistors R2, R3, and R4 are shared by the oscillating circuit 2 and the buffer circuit 3.
In the frequency-switching oscillator 1 configured as above, the resonant frequency in the resonant circuit 4 is switched between two frequencies. According to this, an oscillation frequency in the oscillating circuit 2 is switched between two frequencies, and the two frequencies are amplified by the buffer circuit 3 so as to be outputted.
In the above configuration, however, characteristics of the matching circuit 7 in the buffer circuit 3 are fixed. Therefore, for the frequency-switching oscillator 1, the matching circuit 7 must be designed so as to match the two oscillation frequencies. However, when the difference between the two frequencies is large, it is difficult for the matching circuit 7, because characteristics are fixed, to handle both frequencies. In this case, problems are that the matching circuit 7 can match only one of the frequencies, and the level of a signal of the other oscillation frequency decreases, thereby disabling signals of substantially the same levels at all the oscillation frequencies to be outputted.
To solve the above problem, an object of the present invention is to provide a frequency-switching oscillator and a communication apparatus using the frequency-switching oscillator. Corresponding to frequency-switching in an oscillating circuit, a frequency-switching oscillator to be provided can switch matching frequencies in a buffer circuit so as to produce outputs of substantially the same levels at any of the frequencies.
To these ends, according to one aspect of the present invention, there is provided a frequency-switching oscillator having an oscillating circuit that has a first active element and that can switch and output a plurality of oscillation frequencies which are different from each other; and a buffer circuit, the buffer circuit is connected to an output of the oscillating circuit and comprises a second active element and a matching circuit. In the above, the matching circuit has a diode, the anode of the diode is switched between a first potential and a second potential which is higher than the first potential, and the cathode of the diode is set to a value of a third potential which is higher than the first potential by a predetermined level and is lower than the second potential by a switch voltage level or greater of the diode.
According to the above configuration, corresponding to oscillation-frequency switching by the oscillating circuit, frequencies to match the buffer circuit are switched, and outputs of substantially the same levels for any of the oscillation frequencies can be obtained.
In the above case, the first active element and the second active element may be cascade-connected, a connection point of the first and second active elements may be grounded via a capacitor in an RF band to be used (in a high frequency band to be used), and the cathode of the diode may be connected to the connection point of the first and second active elements. This allows reduction in the number of component parts, thereby allowing miniaturization and cost reduction.
Also, in the frequency switching oscillator of the present invention, one of the terminals of the second active element may be grounded in an RF band to be used via a capacitor, and the cathode of the diode is connected to a connection point of the second active element and the capacitor. This also allows reduction in the number of component parts, thereby allowing miniaturization and cost reduction.
Furthermore, according to another aspect of the present invention, there is provided a communication apparatus using the frequency-switching oscillator described above. Because of using the frequency-switching oscillator described above, the communication apparatus allows multiple frequencies to be transmitted and received stably.